Are the images hosted on your domain?
The timthumb script requires that all images being resized be hosted on the same domain as your WordPress blog. This is a security measure put in place by the author. If you are trying to use images from an outside sources it will not work.
Does your cache folder have the correct permissions?
You need to give your /cache/ folder full permissions. In most cases this is “777” but it may vary depending on where you are hosted. If 777 does not work, try 755. It is recommended that you contact your host about what permissions to use if you are not sure. Settings permissions is also known as CHMODing, and can be done using most FTP programs and hosting control panels.
Is your image extension valid?
A few times I have seen users that have images with the wrong extensions. For example, a PNG image has been saved with a .JPG extension. This can confuse the script. Make sure that your image have the correct extensions. To figure out what extension it should have, type the URL of your image in your browser and view the image. Right click it and choose “Properties.”
Does your server have the GD library compiled with its version of PHP?
This script requires the GD library to be installed. If your thumbnails are not working, check with your host to make sure that you have it.
Does your server have mod_security settings that are impeding the scripts functionality?
Some servers may have mod_security settings that will stop the script from working. You should always contact your host if you are having problems to make sure they do not have settings enabled that would stop the script from working.
Is the image you are trying to resize HUGE?
If the source image for your thumbnail is very large in resolution your server might not have enough memory to create the cached image. This often occurs when people try to use a 12 megapixel photo from their camera. Try resizing your image down to around 500px in width before adding it to the custom field if you are experiencing problems.